Run-length-limited variable-length code word data encoding and decoding techniques are well known in the art and are fully discussed in U.S. Pat. No. 3,689,899 to Franaszek, which also includes a full discussion of the advantages of these techniques. Speaking broadly, use of run-length-limited, variable-length code word encoding schemes allows data to be encoded in such a way that it can be recorded at relatively high density on magnetic storage media such as magnetic disks or tape, and also allows the detection of errors in decoding thereof such that only words erroneously decoded are lost. Certain prior techniques did not permit recovery from errors, such that errors could propagate indefinitely. The use of a variable-length code word encoding scheme also allows some substantial simplification of circuitry and higher speed of operation than did fixed-length encoding schemes. For these reasons, run-length-limited, variable-length code word encoding schemes, such as proposed in the Franaszek patent, have become near universal in the art.
According to the run-length-limited, variable-length encoding schemes proposed in the Franaszek patent, the codes proposed can be referred to generally as (d,k) codes, where d is the minimum number of "zeroes" between any two adjacent "ones" in an encoded data pattern, and k is the maximum number of zeroes between any two adjacent "ones". A minimum number of zeroes is required so that the adjacent ones are always separated on the media by some minimum distance required to avoid distortion and the maximum number is required so that the ones are never separated by more than a given distance, so that self-clocking of the data is still possible. It will be understood by those skilled in the art that according to modern recording techniques only the ones are actually recorded on the media; self-clocking is used upon decoding to insert the correct number of zeroes between adjacent ones in the decoded data stream.
Use of such run-length-limited (d,k) codes, in which d=2 and k=7, are nearly universal in the data processing art today, and numerous patents show circuitry for achieving (2,7) encoding and decoding. Eggenburger U.S. Pat. No. 4,115,768 shows a typical approach, while co-pending U.S. patent application Ser. No. 444,158, filed Nov. 24, 1982 on behalf of the present inventor shows an improvement on the circuitry shown in the Eggenburger patent.
The Franaszek patent also discusses a code in which d and k are equal to 1 and 8 respectively, that is in which adjacent "ones" in an encoded data stream are separated by no less than one nor no more than eight "zeroes." However, the circuitry proposed by the Franaszek patent is in block diagram form only, particularly as regards the decoder; as far as the present inventor knows, no circuit has been fully designed which implements the (1,8) code proposed by Franaszek. Moreover, as far as the present inventor is aware no commercial system has been built which uses this code.
Use of the (1,8) code has several advantages over the (2,7) code, chiefly higher data rate. For instance, a code word of 6 bits written on a disk represents 4 data bits in the (1,8) scheme but only 3 bits in a (2,7) scheme. Assuming the number of encoded bits per inch remains the same, the (1,8) code therefore allows an increase of 33% in the effective data density. In the highly competitive data processing market, a 33% improvement is very substantial. It should be recognized, and will be by those skilled in the art, however, that the condition that the number of encoded bits written per inch remains the same is not trivial because according to the (1,8) code, d, the minimum distance between adjacent ones, is only half that of the (2,7) code. According to the 2,7 codes, the closest flux transitions are thus separated by two zeroes: 100100 . . . , while in the 1,8 code, the 101010 . . . pattern can appear. Accordingly, for a given minimum spacing between flux changes, for a given head/media combination, the net uncoded bit density remains the same. Closeness of adjacent transitions (representing ones) is a limiting factor in disk drive performance. Nevertheless, improvements in technology are being made continually, and it seems highly likely that soon the (1,8) code will be highly desirable if not commercially requisite.
Even if such improvements in the maximum flux change density permissible with a given head/disk combination are not forthcoming, the 1,8 code has an advantage over the 2,7 code in the area of the "detect window". That is, data encoded by 1,8 rules is somewhat easier to read from the disk. Accordingly, there is a need in the art for a high speed circuit for encoding and decoding magnetic data according to the (1,8) run-length-limited variable-length coding rules shown in the Franaszek patent. Such a circuit to be successful would have to be capable of reliably encoding and decoding data at very high rates up to on the order of 40 MHz. Furthermore, such circuits would desirably not be prohibitively complex, nor expensive to manufacture.